Taking Payments Via Paypal Coding Help

I have created a site that gives the user a custom URL and then they can choose to purchase it. Is there anyway to pass that url through paypal so that I can see which one they are actually wanting to buy? Right now I have them put the URL in the seller instructions, but some forget and im sure it turns others off having to add an extra step. Ive done some research but I cant find a good answer.

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